/Engine/Source/Runtime/Core/Public/Containers/Map.h
Add all items from another map to our map (if any keys are in both, the value from the other map wins).
template<typename OtherSetAllocator> void Append ( const [TMultiMap](API\Runtime\Core\Containers\TMultiMap)< KeyType, ValueType, OtherSetAllocator, KeyFuncs > & OtherMultiMap )